DESCRIPTION

The event API provides a mechanism to execute a function when a specific
event on a file descriptor occurs or after a given time has passed.
The event API needs to be initialized with event_init() before it can be
used.
In order to process events, an application needs to call
event_dispatch(). This function only returns on error, and should replace
the event core of the application program.
The function event_set() prepares the event structure ev to be used in
future calls to event_add() and event_del(). The event will be prepared
to call the function specified by the fn argument with an int argument
indicating the file descriptor, a short argument indicating the type of
event, and a void * argument given in the arg argument. The fd indicates
the file descriptor that should be monitored for events. The events can
be either EV_READ, EV_WRITE, or both, indicating that an application can
read or write from the file descriptor respectively without blocking.
The function fn will be called with the file descriptor that triggered
the event and the type of event which will be either EV_TIMEOUT,
EV_SIGNAL, EV_READ, or EV_WRITE. Additionally, an event which has re-
gistered interest in more than one of the preceding events, via bitwise-
OR to event_set(), can provide its callback function with a bitwise-OR of
more than one triggered event. The additional flag EV_PERSIST makes an
event_add() persistent until event_del() has been called.
Once initialized, the ev structure can be used repeatedly with
event_add() and event_del() and does not need to be reinitialized unless
the function called and/or the argument to it are to be changed. However,
when an ev structure has been added to libevent using event_add() the
structure must persist until the event occurs (assuming EV_PERSIST is not
set) or is removed using event_del(). You may not reuse the same ev
structure for multiple monitored descriptors; each descriptor needs its
own ev.
The function event_add() schedules the execution of the ev event when the
event specified in event_set() occurs or in at least the time specified
in the tv. If tv is NULL, no timeout occurs and the function will only be
called if a matching event occurs on the file descriptor. The event in
the ev argument must be already initialized by event_set() and may not be
used in calls to event_set() until it has timed out or been removed with
event_del(). If the event in the ev argument already has a scheduled
timeout, the old timeout will be replaced by the new one.
The function event_del() will cancel the event in the argument ev. If the
event has already executed or has never been added the call will have no
effect.
The functions evtimer_set(), evtimer_add(), evtimer_del(),
evtimer_initialized(), and evtimer_pending() are abbreviations for common
situations where only a timeout is required. The file descriptor passed
will be -1, and the event type will be EV_TIMEOUT.
The functions signal_set(), signal_add(), signal_del(),
signal_initialized(), and signal_pending() are abbreviations. The event
type will be a persistent EV_SIGNAL. That means signal_set() adds
EV_PERSIST.
The function event_once() is similar to event_set(). However, it
schedules a callback to be called exactly once and does not require the
caller to prepare an event structure. This function supports EV_TIMEOUT,
EV_READ, and EV_WRITE.
The event_pending() function can be used to check if the event specified
by event is pending to run. If EV_TIMEOUT was specified and tv is not
NULL, the expiration time of the event will be returned in tv.
The event_initialized() macro can be used to check if an event has been
initialized.
The event_loop function provides an interface for single pass execution
of pending events. The flags EVLOOP_ONCE and EVLOOP_NONBLOCK are recog-
nized. The event_loopexit function exits from the event loop. The next
event_loop() iteration after the given timer expires will complete nor-
mally (handling all queued events) then exit without blocking for events
again. Subsequent invocations of event_loop() will proceed normally. The
event_loopbreak function exits from the event loop immediately.
event_loop() will abort after the next event is completed;
event_loopbreak() is typically invoked from this event's callback. This
behavior is analogous to the "break;" statement. Subsequent invocations
of event_loop() will proceed normally.
It is the responsibility of the caller to provide these functions with
pre-allocated event structures.

EVENT PRIORITIES

By default libevent schedules all active events with the same priority.
However, sometimes it is desirable to process some events with a higher
priority than others. For that reason, libevent supports strict priority
queues. Active events with a lower priority are always processed before
events with a higher priority.
The number of different priorities can be set initially with the
event_priority_init() function. This function should be called before the
first call to event_dispatch(). The event_priority_set() function can be
used to assign a priority to an event. By default, libevent assigns the
middle priority to all events unless their priority is explicitly set.

THREAD SAFE EVENTS

Libevent has experimental support for thread-safe events. When initializ-
ing the library via event_init(), an event base is returned. This event
base can be used in conjunction with calls to event_base_set(),
event_base_dispatch(), event_base_loop(), event_base_loopexit(),
bufferevent_base_set() and event_base_free(). event_base_set() should be
called after preparing an event with event_set(), as event_set() assigns
the provided event to the most recently created event base.
bufferevent_base_set() should be called after preparing a bufferevent
with bufferevent_new(). event_base_free() should be used to free memory
associated with the event base when it is no longer needed.

BUFFERED EVENTS

libevent provides an abstraction on top of the regular event callbacks.
This abstraction is called a buffered event. A buffered event provides
input and output buffers that get filled and drained automatically. The
user of a buffered event no longer deals directly with the IO, but in-
stead is reading from input and writing to output buffers.
A new bufferevent is created by bufferevent_new(). The parameter fd
specifies the file descriptor from which data is read and written to.
This file descriptor is not allowed to be a pipe(2). The next three
parameters are callbacks. The read and write callback have the following
form: void(*cb)(struct bufferevent *bufev, void *arg). The error call-
back has the following form: void(*cb)(struct bufferevent *bufev, shortwhat, void *arg). The argument is specified by the fourth parameter
cbarg. A bufferevent struct pointer is returned on success, NULL on er-
ror. Both the read and the write callback may be NULL. The error callback
has to be always provided.
Once initialized, the bufferevent structure can be used repeatedly with
bufferevent_enable() and bufferevent_disable(). The flags parameter can
be a combination of EV_READ and EV_WRITE. When read enabled the buf-
ferevent will try to read from the file descriptor and call the read
callback. The write callback is executed whenever the output buffer is
drained below the write low watermark, which is 0 by default.
The bufferevent_write() function can be used to write data to the file
descriptor. The data is appended to the output buffer and written to the
descriptor automatically as it becomes available for writing.
bufferevent_write() returns 0 on success or -1 on failure. The
bufferevent_read() function is used to read data from the input buffer,
returning the amount of data read.
If multiple bases are in use, bufferevent_base_set() must be called be-
fore enabling the bufferevent for the first time.

ADDITIONAL NOTES

It is possible to disable support for kqueue, poll or select by setting
the environment variable EVENT_NOKQUEUE, EVENT_NOPOLL or EVENT_NOSELECT,
respectively. By setting the environment variable EVENT_SHOW_METHOD,
libevent displays the kernel notification method that it uses.

RETURN VALUES

Upon successful completion event_add() and event_del() return 0. Other-
wise, -1 is returned and the global variable errno is set to indicate the
error.

SEE ALSO

HISTORY

The event API manpage is based on the timeout(9) manpage by Artur Gra-
bowski. Support for real-time signals was added by Taral.

AUTHORS

The event library was written by Niels Provos.

BUGS

This documentation is neither complete nor authoritative. If you are in
doubt about the usage of this API then check the source code to find out
how it works, write up the missing piece of documentation and send it to
me for inclusion in this man page.
MirOS BSD #10-current October 19, 2012 5